Prostate cancer (PCa) is a clinically heterogeneous disease, with marked variability in patient outcomes. Striking molecular heterogeneity may underlie the variable clinical course, with distinct molecular subtypes recently identified. However, the biologic implications and translational impact of novel subtypes are largely undefined, and there is a critical need for new models to study the biology and therapeutic vulnerabilities of these novel subtypes of PCa. Recurrent mutations in SPOP occur in about 10% of PCa, and define a distinct, core molecular class of PCa. However, the mechanisms by which SPOP mutation drives prostate tumorigenesis in vivo remain unknown. Using multiple novel in vitro and in vivo models of SPOP mutant PCa, preliminary data demonstrate that SPOP mutation drives neoplasia and invasive cancer in the mouse prostate. Furthermore, preliminary studies in models in vitro and in vivo support that SPOP mutation activates the phospho-inositide 3-kinase (PI3K) signaling pathway, with evidence for this regulation in human PCa. Interestingly, while PI3K activation normally downregulates androgen receptor (AR) signaling, we show that SPOP mutation interrupts this negative feedback by upregulating AR. These results suggest that SPOP mutation activates two known critical signaling pathways in PCa ? AR and PI3K signaling. Based on these preliminary findings, we hypothesize that SPOP mutation drives prostate tumorigenesis through coordinate regulation of AR and PI3K signaling pathways. To address this hypothesis, we will utilize novel in vitro and in vivo models and human PCa samples to establish the mechanistic basis of SPOP regulation of PI3K/mTOR signaling, define the relative importance of PI3K/mTOR and AR signaling in driving formation and progression of SPOP mutant PCa, and determine the potential for therapeutic targeting of AR and PI3K/mTOR signaling in SPOP mutant PCa. This project leverages unique in vivo and in vitro model systems to study a recently discovered key subtype of PCa. Together, these studies will establish the importance of PI3K signaling and AR signaling in PCa driven by SPOP mutations, define the critical signaling events in SPOP mutant PCa and the broader applicability across PCa subtypes, and provide the foundation for clinical trials targeting this subclass.